Local-Use Addresses

A local-use address is a unicast address that has only local routability
scope. A local-use address can only be used within the subnet or within a
subscriber network. These addresses are intended for use inside of a site
for plug and play local communication and for bootstrap
operations for the use of global addresses.

The two types of local-use unicast addresses are link-local and site-local.
The Link-Local-Use is for use on a single link. The Site-Local-Use is for
use on a single site. The following table shows the Link-Local-Use address
format.

Table 14–3 Link-Local-Use Addresses Format

10 bits

54 bits

64 bits

1111111010

0

Interface ID

Link-Local-Use addresses
are used for addressing on a single link for purposes such as auto-address
configuration.

The following table shows the Site-Local-Use address format.

Table 14–4 Site-Local-Use Addresses

10 bits

38 bits

16 bits

64 bits

1111111011

0

Subnet ID

Interface ID

For both types of local-use addresses, the
Interface ID is an identifier that must be unique in its domain. In most instances,
the identifier uses a node's IEEE-802 48–bit address. The Subnet ID
identifies a specific subnet in a site. The Subnet ID and the interface ID
form a local-use address. Consequently, a large private internet can be constructed
without any other address allocation.

Organizations that are not yet connected to the global Internet can
use local-use addresses. Local-use addresses enable organizations to operate
without the need to request an address prefix from the global Internet address
space. If the organization later connects to the Internet, the Subnet ID,
Interface ID, and a global prefix can be used to create a global address.
For example, the organization can use the Registry ID, Provider ID, and the
Subscriber ID to create a global address. This enhancement is a significant
improvement over IPv4. IPv4 requires sites that use private (non-global) IPv4
addresses to manually renumber when sites connect to the Internet. IPv6 automatically
does the renumbering.